Incandescent lamps are unity power factor devices where the current is proportional to instantaneous line voltage, and line current is always present, except at zero cross points. The zero cross points are the points on the voltage wave where the line voltage is zero. Solid state relays, such as AC thyristor switches, are useful for controlling large AC power flows with milliampere-scale control circuits. AC thyristors can be used with incandescent lamps because there is always enough current to keep the switches from turning off except at the zero cross over of the line voltage. A trigger pulse of current applied shortly after the zero cross over will turn the thyristor on again for powering incandescent lamps.
Newer high-efficiency lamps, such as compact fluorescent and LED lamps, consume electricity differently from traditional incandescent lamps, drawing their currents in short erratic bursts (non-linear loading) rather than smoothly (linear loading). Most non-incandescent lamps include an electronic ballast which uses an alternating current to direct current converter circuit which interrupts the current through its switching action. These types of circuits normally only conduct the line current for a brief time within the instantaneous line voltage maximum and minimum peaks. Non-linear loads change the shape of the current waveform from a sine wave to some other distorted form, creating harmonic currents in addition to the original (fundamental frequency) AC current. These harmonic waveforms are a distortion of the current waveform, and change the phase relationship between the voltage and the current. Thus, as non-incandescent lamps can have their voltage and current waveforms out of phase, they are difficult to control using AC thyristor switches.
For most non-incandescent lamps, the distortion of the current waveform causes there to be no line current available shortly after the zero cross over of the line voltage, resulting in the AC thyristor not activating. One solution is to design a delay of the trigger pulse until current is present for a specific non-incandescent lamp waveform. While the trigger pulse can be controlled to delay until a point where current is present in the harmonic waveform, different non-incandescent lamps can have different phase angle relationships between the voltage and the current, where they start conducting current, thus necessitating a variety of trigger delay points.
Non-incandescent lamps can be controlled by photosensitive devices, such as photodiodes, phototransistors, and photodarlington light sensors. However, light given off by the controlled lamp can be reflected off nearby objects, causing the lamp to oscillate on and off. Compact fluorescent and LED lamps, unlike incandescent lamps, emit very little light in the infrared spectrum. Sunlight, conversely, contains a large amount of infrared light. Photosensitive devices with an infrared filter are useful to conduct only the infrared wavelength light.
A photo controller, a photo controller assembly, and a process for controlling a non-unity power factor device not suffering from one or more of the above drawbacks would be desirable in the art.